The PX4 is a directly heated audio power triode valve introduced by Osram in 1929. . The classic electrode cage possesses a single filament wire formed into four cantilever sprung inverted V forms situated within the centre of the cage. The grid is similarly single wire supported by two vertical notched rods 1.5 mm in diameter and zigzags around the filament.
It’s introduction sparked a scuffle amongst contemporary valve manufacturers whom scrambled to get their own versions to market, first we had Mullard introducing the AC044 in 1930, then Mazda introducing the PP3/250 in 1932 and Ferranti getting into the act in 1933 with the LP4.
These valve types underwent various form and performance changes over the years and the early globe envelope devices are keenly sought even though their dissipation was 12W in class AB1 push-pull. The addition of fins orthogonally added to the anode gave a handy dissipation increase to 15W and addition of an ultra black carbonized anode coating gave up 16W with some of the tall form coke bottle variants capable of delivering a healthy 20W dissipation.
Aesthetics and power handling aside, any of these variants are highly prized superlative performers capable of an exquisite sound stage presentation - indeed the king of valves!